Good evening! I'm pretty new to stacking and I have encountered a bit of a problem, I think.

The image doesn't seem sharp enough, what am I doing wrong?

First of all, the setup.

I use a wooden board, with a QD-plate which supports the WEMacro-rig, the PB6-belows with my camera(Nikon D7100) and the lens (with diffuser). I use 2x Jansö lamps and one Meike MK-300 flash. See picture.

I found a bug of some sort and decided to give it a try with 2 different microlenses. The first one is a Nikon BD Plan 10x (na0,25) 210/0. Which gave me this picture:

It looks OK, but not nearly as sharp as I would "prefer" so to speak. When browsing the forum I see knife sharp images and this one is.. Well. Dull?

So, the real question(s) here is:

How do I improve the sharness with what I have? On this particular photo I used the Meike flash (1/128) (in combination with 2 Jansjö lamps) with sync speed 250.
I used the mirror lock up function in the camera as well. A plastic cup served as a diffuser. Is the lack of sharpness due to microvibrations? Not enough light? Or is it the lens?

I edited the photo in ZS with Dmax and used Pmax to clean some hairs and so on. Sharpened in PS and adjusted the clarity.

I tried to make a 100% crop, but it became even worse(?)

I took another shot at the bug with my second lens (a 20x noname from surplusshed) with same settings (except extension), which resulted in this image:

I still think it's a OK picture, but I still think it's a bit soft. Sharpened and some adjustments in camera-raw in PS.

How shall I proceed? Another flash? I've tried a pingpong ball as a diffuser, but it gave me a white image, I can't reposition the flash far back enough as it is just now.

I hope I made some kind of sense here. If not, please ask and I will try to get as much details as you need.

Your subject appears to be a lacewing, either one from the "brown lacewing" family Hemerobiidae, or a "green lacewing" (family Chrysopidae) in a brown color phase.

I note that you say "new to stacking" and that you're using microscope objectives.

That combination often produces an unrealistic expectation about sharpness.

At 10X, an NA 0.25 objective is running at f/20 effective aperture back at the camera sensor. With a Nikon D7100, at 24.1 megapixels on an APS-C sensor, f/20 is firmly in diffraction territory. The camera sensor has a little more resolution than the optical image does, and even at the scale of image elements that span several pixels, the image contrast is getting cut way down just because of diffraction blurring. Whatever aberration the lens has just makes things worse.

So it is not physically possible for the optical image to look sharp when viewed at the scale of individual pixels, as in a 100% crop.

In the images shown here, I do not see any obvious defects that are affecting sharpness. I see no trace of motion blur.

I notice in the first image that there are no good blacks, so I agree with DesolateMirror that you probably have some flare issues. Working to reduce those is good, but they are hard to completely eliminate with microscope objectives. To my eye, significant improvement can be made to the image that you already have, just by applying a curves/levels adjustment to stretch the histogram down to include good blacks.

Now I need to figure out how to get rid of the halo sorrounding the subject.

In the current picture, a lot of what I would call halo appears to be out-of-focus portions of the subject, seen by the objective looking around the edge of the subject. At NA 0.25, an objective actually sees about 15 degrees behind any edge, due to the width of the entrance cone. As a result, when you think the objective is focused on empty space and seeing just dark background behind it, the objective can actually be seeing other parts of the subject too.

To get a better handle on this effect, I suggest trying deeper stacks, deep enough to catch all of the subject in focus, at least in some of those areas that look haloed. Then you can at least look back to the original source frames to diagnose what happened in the stacked output.

PMax can cause another sort of halo, which usually manifests as irregular blotches in what should be uniform background, next to a strongly contrasting subject. Those can be avoided by using DMap with a properly chosen contrast threshold. In your earlier 20X shot, I'm pretty sure I see some of these in the background to the left of the eye. In your current 10X shot I do not see any of these PMax halos.

By the way, it is common for discussions about image defects like halo to get confused because people are actually talking about different defects, but thinking that they're talking about the same thing. When in doubt, it is best to mark up the image to be discussed, to clearly indicate the areas where you see problems.